There are many advantages to cryogenic freezing of food products which have come to light in the past several decades, and in a number of instances, carbon dioxide is the cryogen of choice for efficient and economical cryogenic freezing applications. Cryogenic carbon dioxide food freezers often utilize liquid carbon dioxide under pressure sufficient to maintain it in the liquid state and supply it to spray nozzles through which it is injected into the interior of a thermally insulated enclosure wherein the food products to be frozen are delivered to a freezing region, as by being transported on an endless conveyor or the like. In a CO.sub.2 food freezer, the low temperatures which can be achieved by the creation of solid CO.sub.2, can create a tendency for liquid CO.sub.2 in the lines leading to the spray nozzles to freeze, particularly at times when there is no flow or only very low flow. Gassing systems have been devised and utilized to clear the lines of liquid CO.sub.2 at certain times to prevent such freezing.
Freezing of food products is typically accomplished by heat transfer to the colder gas that is being circulated past the food products, although some heat may be withdrawn by direct removal to a vaporizing cryogen at the surface of such a food product. Accordingly, the movement of the gas and its velocity become important in accomplishing efficient freezing of the food products, and one or more blowers is generally always provided to assure the food products are exposed to and in contact with the circulating cold vapor.
U.S. Pat. No 4,356,707, in FIGS. 10-12, shows cabinet freezers including one having a spiral or helical conveyor wherein CO.sub.2 injectors fed from an upper liquid CO.sub.2 source are located in corner regions of the cabinet to inject CO.sub.2 snow and cold vapor and to induce additional vapor flow generally horizontally and in a direction concurrent with the movement of the food products along the helical path. U.S. Pat. No. 4,324,110 shows a cryogenic food freezer wherein liquid CO.sub.2 from an upper supply line 38 is injected through nozzles and discharged countercurrently into streams of moving gas or vapor from fans to effect rapid vaporization of the injected CO.sub.2. U.S. Pat. No. 3,733,848 shows a food freezer wherein a header 82 extending along the roof of a freezer enclosure supplies spray nozzles that inject CO.sub.2 into discharge streams from vertically elongated blowers having vane-carrying squirrel cage rotors which rotate about vertical axes. U.S. Pat. No. 4,078,394 shows a spiral freezer designed for cold gas to flow through the various regions of a helical belt by driving a center drum of circular cross-section having both its axial ends open and having a varied perforation pattern in its sidewall wherein gas sucked from the interior of the drum by a motor-driven is fan is discharged past a plurality of injectors in the top wall of the freezer where CO.sub.2 vapor is injected to effect cooling of the gaseous atmosphere.
When liquid CO.sub.2 is injected through spray nozzles located within a thermally insulated enclosure, it is possible for solid cryogen to begin to form in the lines leading to the nozzles as by freezing liquid CO.sub.2 which is in contact with the cold metal surfaces of the lines within such cold environment and/or the nozzles, which can cause the nozzle orifices to clog. Moreover, there is always the possibility that small amounts of CO.sub.2 snow will also form in the lines through momentary pressure drops, and because of the cold environment, such snow will be relatively slow to redissolve in liquid CO.sub.2 and will generally be carried along to the snow nozzles where buildup and blockage can occur.
Inefficiencies result from such clogged spray nozzles in a freezer, and accordingly, solutions to such problems were sought.